Ceramic materials such as sheets and fabrics comprised of ceria (CeO2), zirconia (ZrO2) and alumina (Al2O3) have been used in high temperature sealing and refractory applications. In particular, such ceramic materials have been used to manufacture conventional gaskets for wet and/or dry sealing of various high temperature fluids. However, such conventional gaskets are pliable and tend to sag when handled or otherwise manipulated during an assembly process. In addition, conventional gaskets have relatively low compressive strength. For example, conventional gaskets can irreversibly crush and achieve a strain of almost 0.9 in/in, when subject to a relatively low compressive load, thereby degrading the gaskets' sealing performance.
Gaskets made from ceramic materials have also been used in various fuel cell applications. Conventional fuel cells typically include an anode and a cathode separated by an electrolyte contained in an electrolyte matrix. The anode, the cathode, the electrolyte and the electrolyte matrix are disposed between a first collector and a second collector, with the first collector adjacent to the anode and the second collector adjacent to the cathode. Fuel flows to the anode via the first collector and an oxidant flows to the cathode via the second collector. The fuel cell oxidizes the fuel in an electrochemical reaction which releases a flow of electrons between the anode and cathode, thereby converting chemical energy into electrical energy.
The fuel cells described above can be stacked in series with separator plates disposed between adjacent fuel cells and end plates disposed on opposing ends of the fuel cell stack. Fuel cells are stacked to increase the electrical energy they produce. Fuel cell stacks have a negative side with a negative end cell and a positive side with a positive end cell.
One particularly useful fuel cell stack is a Molten Carbonate Fuel Cell (MCFC) stack which generally has an operating temperature of approximately 650 degrees Celsius. A problem sometimes experienced by liquid phase fuel cells such as MCFCs is electrolyte migration which is characterized by the loss of the electrolyte from one end cell and the gain of electrolyte by the other end cell. Electrolyte migration is caused by an electric voltage gradient along the length of the stack and generated by the cells in the MCFC stack. Loss of the electrolyte from the positive end cells can cause gas pockets in the electrolyte matrix of the positive end cell. This results in an irreversible increase in internal electrical resistance causing a significant voltage drop across the positive end cell and therefore decreasing the useful life of the MCFC stack. Migration of the electrolyte towards the negative end cell can also cause the negative end cell to become flooded with electrolyte, thereby reducing MCFC stack performance and life.
Electrolyte migration can occur because the electrolyte is a molten liquid when the MCFC is at its operating temperature. Thus during operation, when the electrolyte is liquid, the electrolyte can flow along an outer surface of the MCFC stack. In particular, the electrolyte can flow in and/or under a gasket disposed between the outer surface and a manifold used to channel fluid such as fuel and air to the fuel cell. Problems with conventional ceramic gaskets (e.g., felts of zirconia, alumina and ceria) used for liquid phase fuel cells include: absorbing high amounts of electrolyte, acting as a conduit for electrolyte movement and having low strength. The low strength of these materials makes them difficult to handle and install in fuel cells. Ceramic gaskets such as those consisting of ZYF100 zirconia felt manufactured and as received from by Zircar Zirconia, Inc. of Florida, N.Y. have been used as a material for MCFC gaskets. However, such gaskets typically can cause performance problems associated with electrolyte migration and have poor mechanical properties (e.g., low compressive strength and significant sag).
Attempts have been made to improve the mechanical properties of ceramic fabric through heat treatment, but only limited success has been achieved. For example, heat treatment of ceramic fabric on refractory brick causes the ceramic fabric to react with and/or be absorbed by the refractory brick. In addition, attempts to heat treat ceramic fabric on alumina blocks having a 96 percent by weight purity resulted in the ceramic fabric sticking to the alumina block.
Attempts have also been made to identify ceramic gasket materials with reduced electrolyte absorption to reduce electrolyte migration in MCFCs. However, the conventional zirconia and alumina gasket materials have high electrolyte absorption and undesirable migration rates. Furthermore, alumina is shown to be unstable and reacts with molten alkali carbonate electrolyte to form LiAlO2.